The best way to get good at any such discipline is to learn from the best practitioners you have access to.
However, in addition to what I said above ^^^ the rule in question allows the school/gym/dojo to control the speed of your development. They make more money that way. If you start out practicing with higher belt levels, you will soon be at a level in skill that surpasses the belt you've been awarded, and that creates conflict and political problems. Most traditionally structured martial arts classes are NOT geared toward individuals any more than military boot camp is.
You want to see a big stink? Walk into your martial arts class, whatever it may be, and if it's belt oriented, tell them you're not gonna bother with that aspect anymore but you still want to learn all you can.
Luckily, there's things like MT and catch/sub wrestling that favor that sort of atmosphere.